Bicomponent ceramic fibers

ABSTRACT

A continuous, bicomponent, non-vitreous ceramic fiber comprises components existing in a longitudinal side by side relationship wherein each of the components is derived from a different fiber-forming precursor liquid. Firing the bicomponent fibers in a reducing atmosphere can provide ceramic/cermet or cermet/cermet fibers wherein each cermet component has a graded composition.

FIELD OF THE INVENTION

This invention relates to bicomponent, non-vitreous ceramic fiberswherein the component fibers are in a continuous, longitudinallyextending, side-by-side configuration. In another aspect, it relates toa process for preparing bicomponent ceramic fibers from two distinctprecursor liquids. In a further aspect, the bicomponent ceramic fiberscan be converted to cermet/cermet or ceramic/cermet fibers.

BACKGROUND ART

Within the last decade, an amount of literature has been publisheddescribing various polycrystalline, microcrystalline, or non-vitreousfibers and other shaped articles of refractory metal oxides. Thesearticles are made by various non-melt processes, such as by drying filmsof solutions of metal oxide precursors or oxide sols, or drying organicpolymeric bodies, such as cellulose or rayon, impregnated with such asolution, or by extruding and drawing, or spinning, viscous fluids ofsuch metal compounds into fibers. The fibers are then heated to removewater, organic material, and other volatile material to produce arefractory article. A review of the state of the art of polycrystallineinorganic fibers appears in Chapter 8 of "Modern Composite Materials"edited by Broutman and Krock, published by Addison-Wesley Pub. Co.,Reading, Mass. (1967). Other art in this area is Netherlands Pat. No.7,015,245, British Pat. No. 1,287,288, U.S. Pat. Nos. 3,385,915,3,632,709, 3,663,182 and the art cited in U.S. Pat. No. 3,709,706. Oxidefibers other than those identified as fiberglass are still in therelatively early stage of development. In many technologies, there is aneed for a relatively inexpensive continuous refractory fiber productwith desirable physical properties, such as high strength, high modulusof elasticity, chemical resistance, and the retention of such propertiesafter exposure to high temperatures beyond the capability of presentlycommercially available fiber materials.

Bicomponent fibers are known in the textile art. Typical bicomponentpolymer textiles are disclosed in U.S. Pat. Nos. 4,118,534 and4,278,634. Generally, polymer bicomponent systems relate to two polymersof the same class, e.g., two polyester polymers, or two acrylonitrilepolymers. Usually different polymers in a bicomponent system will splitafter spinning.

Blown-microfibers (3 to 5 micrometer diameter) comprising bicomponentsystems, i.e, polyester-polypropylene have been disclosed in U.S. Ser.Nos. 704,537 and 540,544. Blowing such fibers requires use of a NavalResearch Laboratories dual feed die.

U.S. Pat. No. 2,313,296 teaches concentrically disposed fibers orfilaments of glass. These fibers do not crimp.

It is well-known in the art to prepare monocomponent ceramic fibers fromspinning sols. For example, alumina-silica fibers are disclosed in U.S.Pat. No. 4,047,965; alumina-boria-silica fibers are taught in U.S. Pat.No. 3,795,524; titanium dioxide fibers are disclosed in U.S. Pat. No.4,166,147; zirconia-silica fibers are disclosed in U.S. Pat. No.3,709,706.

SUMMARY OF THE INVENTION

Briefly, the present invention provides a continuous, bicomponent,non-vitreous ceramic fiber wherein the components exist in alongitudinal side-by-side relationship and each of the components isderived from a different fiber-forming precursor liquid.

In another aspect, a spinning process is provided for forming abicomponent ceramic fiber wherein two fiber-forming ceramic liquidprecursor systems of different composition are extruded together throughthe same spinnerette orifices to form a single continuous fibercomprising two conjugate, contiguous ceramic phases in the length of thespun fiber.

The continuous two phase fiber of the present invention preferablycomprises a first phase having a different composition from a secondphase. The two components (two phases) are contiguous in a side by sidearray to form a single ceramic fiber or filament. The present inventioncan provide a high strength carrier component for a low strengthcomponent of special characteristics, for example, magnetic or catalyticcomponents.

The ceramic bicomponent fibers of the present invention are made by anon-melt process comprising shaping viscous concentrates of twoprecursor liquids into a fiber form and then dehydratively orevaporatively gelling or hydrolyizing the drawn or spun fibers. Thesefibers can subsequently be dried to result in a "green" ornon-refractory amorphous fiber. Heating and firing the shaped greenfiber removes water, decomposes and volatilizes undesired fugitiveconstituents, and converts it into the refractory fiber of theinvention.

In this application:

"ceramic" means inorganic nonmetallic material consolidated by theaction of heat, such as metal and nonmetal oxides, carbides, nitrides,sulfides, etc.

"cermet" means a mixture of ceramic and metallic materials;

"sol" means a fluid solution or a colloidal suspension;

"bicomponent fiber" means physically joining together along the lengthof the fibers two components derived from compatible precursor liquidsof different compositions convertible to ceramic materials;

"non-vitreous" means not formed from a melt;

"polycrystalline" means a phase which gives a discernible X-ray powderdiffraction pattern. Crystallite size will affect the line width of theX-ray diffraction pattern. The smaller the crystallite size (belowapproximately 1 micrometer), the broader the lines will become. Thisaffects the resolution of the X-ray pattern fine features such as linesof weak intensity or the ability to separate closely-spaced lines may belost. An overall pattern remains however and is indicative of thecrystal structure;

"microcrystalline" means a crystalline phase having a crystallite orgrain size of about 50 Å to 1000 Å (5×10⁻⁹ to 1×10⁻⁷ m) and sometimeslarger, but always less than 10,000 Å (1×10⁻⁶ m). Such amicroscrystalline structure may be transparent, providing the materialitself is not opaque or contains opaque fillers, large pores, grossareas of inhomogeneity, and the like. Many microscrystalline ceramicsare transparent or translucent;

"amorphous" means a material having a diffuse X-ray diffraction patternwithout definite lines to indicate the presence of a crystallinecomponent;

"dehydrative gelling" or "evaporative gelling", mean that sufficientwater and volatile material are removed from the shaped green fibers sothat the form or shape of the fiber is sufficiently rigid to permithandling or processing without significant loss or distortion of thedesired fibrous form or shape. Therefore, all the water in the shapedfiber need not be removed. Thus, in a sense, this step can be calledpartial dehydrative gelling. The shaped fibers in their green form aregenerally transparent to visible light and clear (or perhaps slightlyhazy) under an optical microscope.

"green" refers to the ceramic articles which are unfired, that is, notin their ceramic form;

"phase" means a component that exists as distinct and separate portionsdistributed throughout a heterogeneous system;

"compatible" means the precursor liquid comprises at least partiallymiscible components; and

"essentially identical" means less than 1 weight percent deviation inany component.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a top plan view of a spinerette that can be used to spinbicomponent fibers in the present invention;

FIG. 2 shows an enlarged fragmentary sectional view taken along the line2--2 of FIG. 1;

FIG. 3 is an elevational view of a bicomponent fiber;

FIG. 4 is an enlarged sectional view taken along line 4--4 of FIG. 3 androtated 90°; and

FIG. 5 is a photomicrograph showing refractory bicomponent fibers of theinvention, fired at 850° C. and enlarged 100 times.

DETAILED DESCRIPTION

The ceramic bicomponent fiber of the present invention can beself-crimping. By controlling the ceramic compositions of the precursorliquids of the two refractory components the degree of crimping may bevaried from a minimal amount to a large amount to provide a high loftcurly fibrous mat. Selection of the sol compositions for finaldifferential densification/shrinkage during the firing process controlsthe amount of crimping.

Preferably the components of the bicomponent fiber are non-vitreousceramics of different compositions. However, it is envisioned within thescope of the present invention that the bicomponent fiber can comprisetwo components of identical final ceramic compositions which werederived from different precursor concentrations. For example, thecomponents of the extruded fiber (green) can be (1) a first componentderived from a first fiberizing batch containing 35 weight percentequivalent oxide, and (2) a second component derived from a secondfiberizing batch containing 25 weight percent equivalent oxide. Thedifference in weight percent equivalent oxide is made up by fugitiveorganic materials. Upon firing, these organic materials will bedifferentially removed, resulting in greater shrinkage of one componentto give a crimped fiber. In this case, the two components of the finalbicomponent fiber have essentially identical ceramic compositions.

Components of the bicomponent fiber are used to produce refractoryfibers known in the art. Bicomponent fibers can be made from precursorliquids such as from zirconia, silica, alumina, titania, chromia, andthoria, as individual sols, or in combination, for example, asalumina-boria-silica, as well as comprising precursor liquids withadditives such as copper, iron, manganese, tin, cobalt, calcium, nickel,tungsten, molybdenum, platinum, and magnetic precursors such as bariumtitanate.

Preparation of aqueous liquid mixtures, sols, or dispersible colloids ormixtures thereof for individual components of the bicomponent fibers ofthe invention are disclosed, for example follows:

    ______________________________________                                        Fiber               U.S. Pat. Nos.                                            ______________________________________                                        titania             4,166,147                                                 alumina-chromia-metal(IV) oxide                                                                   4,125,406                                                 alumina-silica      4,047,965                                                 thoria-silica metal(III) oxide                                                                    3,909,278                                                 aluminum borate and aluminum                                                                      3,795,524                                                 borosilicate                                                                  zirconia-silica     3,793,041                                                                     3,709,706                                                 ______________________________________                                    

In one embodiment, the starting material used to prepare the refractoryfibers of this invention can be prepared where, for example, onecomponent comprises an alumina-silica precursor liquid. An aqueoussilica sol is admixed with a compatible aqueous solution or dispersionof an aluminum compound and, where used, other oxide precursors, such asthe boron and phosphorous compounds, to obtain a uniform dispersionwithout formation of a gel. Generally, this dispersion will be clearthough sometimes it may be hazy. The pH of the dispersion will beinherently on the acid side, e.g., below 6, and is preferably 3 to 5. Ifdesired, a compatible heat fugitive acid, such as acetic or nitric acid,can be added to the silica sol to acidify the same prior to use andprevent premature gelling. Compatible heat fugitive organic agents canbe incorported as adjuvants in the fiber starting material to improveshelf-life of the subsequently concentrated dispersion or to improve thefiberizing nature of the latter. Such organic agents representativelyinclude polyvinylpyrrolidone, polyvinyl alcohol, lactic acid,dextrose/glucose (e.g. corn syrup), and mixtures thereof, theseadditives being oxidized and removed during the firing of the greenfibers produced from such systems.

The aqueous solutions or disperions (precursor liquids) which are usedto make the refractory fibers of this invention optionally can alsocontain various other water-soluble metal compounds (calcinable to metaloxide) which will impart additional desired properties to the refractoryfibers. For example, an optional compound can be used to reduce weightloss, adjust refractive index or dielectric properties, or to impart,without sacrifice of clarity, internal color to the final refractoryupon being converted or oxidized to the corresponding metal oxide. Thus,for alumina-silica, Cr₂ O₃ can be used together with P₂ O₅ to minimizeweight loss otherwise resulting from the latter. Ferric nitrate can beadded to impart an orange to gold color, chromium formate, acetate, ortrioxide to impart to the fibers a green color, cobalt acetate ornitrate to impart a blue or lavender color, vanadyl sulfate to impart ayellow color, nickel acetate to impart a light green to blue color, andmanganese nitrate or acetate to impart a tan to brown color. (Suchcolored refractory fibers, which can be mono- or bi-colored, are usefulfor color coding refractory articles). The ferric oxide-containingrefractory can be reduced in a hydrogen atmosphere, the resultingreduced iron oxide or iron imparting a black color to the refractory andmaking it attractive to a magnet but not electrically conductive. Otheroptional compounds are the water soluble nitrates, formates, acetates,citrates, lactates, tartrates, or oxalates of lithium, sodium,potassium, magnesium, calcium, strontium, barium, yttrium, titanium,zirconium, chromium, manganese, iron, cobalt, nickel, copper, zinc, tin,antimony, lanthanum, and vanadium as vanadyl sulfate.

The amount of such other optional metal oxide in the refractorycomponent can vary, depending upon the property effect desired, e.g.,the tone of the color or hue desired, but generally will be an amount inthe range of as low as 0.05 to 0.5 to as much as 25 weight percent ormore, based on the total weight of the component in the refractoryfiber. Some fiber systems can accommodate these higher amounts of metalcompounds, e.g., the alumina-boria-silica system. The larger amounts ofoptional metal oxide additive may cause the fibers to become friable orgive rise to contamination problems when the fibers are used in aparticular environment. Where these other metal oxides are to be used inalumina-silica fibers having a boria component derived from boric acid,the precursors of the other metal oxides should be other than chloridesif they are used in significant amounts since the combination of boricacid and high chloride levels (which can be determined empirically) inthe starting material generally results in frangible fibers.

Those fibers comprising reducible metal oxides can be further fired in areducing atmosphere, preferably hydrogen, to provide a cermet/cermetfiber or a ceramic/cermet fiber wherein the cermet component is ofgraded composition. The cermet component has a graded ceramic/metalcontent, the metal component decreasing from the periphery towards thecenter of the fiber component with predominant amount of the metal onthe surface, the metal being in the form of discrete, nodular,preferably spheroidal metal particles which can protrude from thesurface of the component and are separated from each other so as toprovide an electrically nonconductive cermet. A "graded cermetcomponent" means one in which the ratio of ceramic/metal is controlledand varied over the thickness of the component; such a cermet mayexhibit a graded or gradual change from less than 100 weight percentmetal content, preferably in the range of 10 to 95 weight percent metalcontent, on the outer surfaces (those surfaces in contact with thereducing atmosphere) to 0 weight percent metal content on the innermostportions of the component.

Each of the fiber precursor materials, as initially prepared, will be arelatively dilute liquid, generally containing about 10 to 30 weightpercent equivalent oxide, carbide, or nitride solids or combinationsthereof, which can be calculated from a knowledge of the equivalentsolids in the raw materials and the amounts thereof used, or determinedby calcining samples of the raw materials or component startingmaterial. For the preparation of fibers, it is necessary to concentrateor viscosify the dilute liquid in order to convert it to a viscous orsyrupy fluid concentrate which will readily gel when the concentrate isfiberized and dehydrated, for example when the concentrate is extrudedand drawn in air to form fibers. The concentration step can be carriedout by techniques known in the art, e.g., see said U.S. Pat. No.3,795,524. Sufficient concentration will be obtained when the equivalentsolids content is generally in the range of 25 to 55 (as determined bycalcining a sample of the concentrate), and viscosities (Brookfield atambient room temperature) are in the range of 10,000 to 1,000,000 cpspreferably 40,000 to 100,000 cps, depending on the type of fiberizing ordehydrative gelling technique and apparatus used and the desired shapeof gelled fiber. High viscosities tend to result in fibers which arecircular in cross-section whereas low viscosities (e.g., less than50,000 cps) tend to result in fibers which are oval or rod-like(elongated ovoid) in cross-section.

In the process for preparing the bicomponent fibers two spinningprecursor liquids, which can be aqueous or organic solutions/sols, ormixtures thereof, are separately prepared as is known in the art. Afterconcentration to viscous, fiberizable concentrates, the separateprecursor liquids are spun together through the same orifice of aspinnerette assembly. The continuous fibers are spun from the dies,which can have about 1 to 21 orifices per centimeter, into a dryingtower from which they are collected and fired. The process is welldescribed above and in U.S. Pat. No. 3,760,049. The orifices of theapparatus are in a row of side-by-side configuration. To insure thestreamline flow of the viscous sol into a bicomponent feed to theorifice, the feed cavity of the die can be optionally fitted with abaffle plate. In a preferred embodiment, no baffle plate is used. Inconducting the spinning, the individual sols, solutions, or mixturethereof, of essentially equivalent rheological properties, particularlyviscosity, are pressured in laminar flow and comingle with onlyinterface mixing at the entrance of the orifice in such a manner as tounite into a single fiber of essentially equal volume proportions. Inthe alternative, bicomponent fibers can be prepared by a methoddescribed in U.S. Pat. No. 4,101,615 in which the green fibers are spunfrom fiberizable organic solutions, followed by hydrolysis, andsubsequent calcination to the ceramic form.

In forming these side-by-side, continuous, bicomponent fibers, it ispreferable to provide essentially equal proportions of the two precursorliquids making up the fiber components. Some degree of variation may bemade by adjustments in viscosities which affects the flow rate of thelaminar flow sols, or proper orifice entry design adjustments in flowcan be made by pressure. It is generally preferred for makingside-by-side bicomponent ceramic fibers that the spinning liquidsentering the orifice should be in essentially a 50:50 volume ratio,preferably 40:60, although a broad range of compositions, for example,10:90 can be useful.

In making continuous fibers, the viscous concentrates can be extrudedthrough a plurality of orifices (e.g., total of 10 to 400) from astationary head and the resulting green fibers allowed to fall in air bythe force of gravity or drawn mechanically in air by means of drawingrolls or a drum or winding device rotating at a speed faster than therate of extrusion. The concentrate can also be extruded through orificesfrom a stationary or rotating head and at the orifice exit blown byparallel, oblique or tangential streams of air, such as in the making ofcotton candy, the resulting blown green fibers being in staple form orshort form with lengths generally 25 cm or less (rather than filamentform) and collected on a screen or the like in the form of a mat. Any ofthese forces exerted on the extruded, green fibers, e.g., gravity,drawing, or air streams, cause attenuation or stretching of the fibers,reducing their diameter by about 50 to 90 percent or more and increasingtheir length by about 300 to 10,000 percent or more and serving tohasten or aid the drying of the green fibers.

The dehydrative gelling of the green fibers can be carried out inambient air, or heated air can be used if desirable or necessary toobtain fast drying. The drying rate assists in controlling of the shapeof the fiber. The relative humidity of such air should be controlledsince large amounts of moisture will cause the gelled or shaped greenfibers to stick together, and excessively dry atmosphere can lead tofiber breakage. Generally, air with relative humidity in the range of 20to 60 percent can be used, at temperatures of 15° to 30° C., though suchair can be heated subsequently to about 70° C. or higher. In some cases,for example, where continuous green fibers are made and gatheredtogether in parallel alignment or juxtaposition in the form of amulti-fiber strand, the fibers or strand can be treated with a size toprevent the fibers from sticking together.

Further detail in fiberizing the viscous concentrate will be omittedhere in the interest of brevity since such procedures are now known,e.g., see said U.S. Pat. No. 3,760,049.

The bicomponent fibers in their green or unfired gel form generallycomprise about 25 to 60 weight percent equivalent oxide, nitride, orcarbide solids (as determined by calcining a sample) and are dry in thesense that they do not adhere or stick to one another or othersubstrates and feel dry to the touch. But the "dry" fibers still containsubstantial amounts of water, organic, and other fugitive material,e.g., 40 to 75 weight percent altogether, and it is necessary to calcineor fire the green fibers in order to remove further water and organicmaterial and convert the fibers into refractory fibers. The term"dehydrative gelling" (or "evaporative gelling"), as used herein,therefore does not mean that all the water in the green fibers isremoved. Thus, in a sense, this step can be called partial dehydrativegelling. It may be noted at this point that the green fibers aretransparent and clear under an optical microscope and, unless coloringadditives are included in the viscous concentrate, they appear to looklike colorless glass fiber. The green fibers are relatively strongenough for further processing and can be collected and fired withoutsignificant breakage.

In order to remove the balance of water and organic material from thegreen bicomponent fibers and convert them to refractory fibers, they arecalcined in a furnace or kiln (preferably an electric resistancefurnace), this heating being carried out usually in air or otheroxidizing atmosphere at temperatures below the fusion or melting pointof the ceramic mixture and usually up to about 800°-1000° C., or incertain cases [e.g., alumina-silica (mullite)] up to 1400° C.Calcination can be accomplished in a number of ways, for example byheating in a single step from a low or room temperature to a desiredelevated temperature (e.g., from room temperture to 1000° C. in 20-60minutes or more) or by heating in a series of steps at progressivelyhigher temperatures, with or without cooling or storage between steps.

The green bicomponent fibers can be calcined in a batch or continuousmanner in an oriented form, such as strands or continous yarn (aplurality of untwisted or slightly twisted parallel-aligned, virtuallyendless, continuous fibers) or hanks (continuous fibers or strands incoiled form), or tows (group of continuous fibers without definite twistand collected in loose form) or calcined in an irregular or randomorder, such as a mat of intermeshed, mechanically interlocked or tangledfibers, or calcined in the form of staple fiber.

In firing the green fibers, care should be exercised to avoid ignitionof combustible material in or evolved from the fibers, for example, bycontrolling the firing atmosphere or by starting out at a lowtemperature, e.g., room temperature, and then elevating the temperatureat a slow rate, since such ignition may cause the formation of opaque,fragile fibers. If the green fibers are not to be fired completely inone operation or are not to be fired immediately or soon after theirformation, it may be desirable or necessary to store the green fibers ina relatively dry or protective atmosphere to prevent them from pickingup moisture or contaminants and deteriorating or sticking together.

The green fibers in their continuous form may be gathered or collectedin the form of a strand, the strand then accumulated in a relaxed,loose, unrestrained configuration of offset or superimposed loops (as ina "figure 8") on a substrate and calcined in that configuration. Incertain cases it may be desirable to pull the strand in a straight orlinear form through a furnace to produce essentially straightenedrefractory strands, a plurality of which can be formed into continuousyarn, all in the continuous manner described in said U.S. Pat. No.3,760,049.

The calcining step volatilizes the balance of the water, decomposes andvolatilizes organic material, and burns off carbon, the resultantrefractory being an essentially carbon-free ceramic refractory. Thiscalcining heating step also causes some shrinkage which is generallyabout 50 percent or more. Shrinkage of one component to a greater extentthan the other results in crimping. However, the shape of the fibersduring firing when so fired are still of essentially continuous lengtheven though they have a high degree of crimp and loft.

The refractory material resulting from firing the green fibers at about900° to 1000° C. comprises crystalline material discernible by x-raypowder diffraction.

The refractory bicomponent fibers of this invention are transparent,glossy, smooth, dense, round, stable, inert, colorless (unless colorantmetal oxide additives are incorporated in the fiber precursor liquid).They can have relatively low weight loss (e.g., less than about 2 weightpercent) and shrinkage (e.g., less than 2.5 linear percent) when heatedor used up to 1100°, and some fibers such as alumina-silica, to 1400° C.They have useful strength, high resistance to fracturing, and areflexible, and can be handled without essentially any breakage. Byflexible is meant the continuous fibers can be bent by hand around arod, e.g., with a diameter of 1.5 mm or a radius or curvature of 0.75mm, without breaking. The properties of the bicomponent fibers aregenerally weighted averages of the components.

In the shaped, fired, refractory bicomponent fibers of the presentinvention each component has at least one microcrystalline phase or canbe amorphous and convertible to at least one microcrystalline phase onfurther firing. The component fibers of this invention may also bepolycrystalline and contain amorphous species. The fibers of thisinvention which have diameters in the range of about 1 to 50micrometers, preferably 5 to 20 micrometers, and for high loftinsulation applications preferably 1 to 5 micrometers, have propertieswhich enable their use in many environments. These fibers may be exposedto high temperatures (e.g., 1000° C. and in certain cases 1400° C.depending upon composition) and may remain strong, flexible andcontinuous.

The fired fibers are continuous, uniformly round or oval, rod-like(elongated ovoid), or ribbon-like, strong, flexible, smooth, glossy,refractory, polycrystalline, or amorphous fibers. The fibers are usefulin making refractory textile fabric or as fillers or reinforcement forplastic composites.

As mentioned above, to spin the bicomponent fibers of the inventionrequires the rheological properties of each precursor liquid besufficiently close to ensure the proper laminar flow into the spineretteorifice to form the fiber. Additionally, the solvents (i.e., (1) wateror water miscible or (2) nonpolar organic solvents such as benzene,dioxane, diethylether, toluene, ethyl n-propyl ether, ethyl isopropylether, tetrahydrofuran, and xylene, or a mixture thereof) and other solcomponents require a degree of compatability to ensure the formation ofa good adhesion interface between the two fiber components.

In one embodiment, as shown in FIGS. 1 and 2, the apparatus 10 andprocedure to make bicomponent fibers 26 of the invention used a singleline of spinnerette holes 12, 152 or 76 micrometer (6 or 3 mil) indiameter and twenty in number. Two precursor spinning liquids 22 and 24were adjusted to about the same viscosity, for example, 50,000 cps. Thepreferred viscosity was about 50,000 to 100,000 cps. The spinningprecursor liquids 22 and 24 were then placed in the spinnerette cup 14.The interface line 28 between the two liquids occurred at the centerlineof the spinnerette orifices 12. Precursor liquids 22 and 24 meet atinterface line 28 but do not mix because of their high viscosities.Because the viscosities of liquids 22 and 24 were essentially the sameand the flow was laminar, the two liquids 22 and 24 arrived at andpassed through orifice 12 at the same flow rate (see FIG. 2). Fiber 26was collected and fired in an air atmosphere furnace from roomtemperature to about 800° to 1,000° C. to produce a refractory firedfiber.

Bicomponent fibers are highly desirable for many uses because suchfibers can be made to be self-crimping and provide woven and nonwovenfabrics or webs of desirable bulk. These properties and others resultfrom the different physical properties of the two spinning precursorsystems which have good adhesion to one another as spun. The fibers areparticularly useful for high temperature, stable, high loft insulation,as catalyst supports, as an open substrate to be infiltrated by resins,glasses, or metals, or mixed with ceramics in a composite.

With the bicomponent spinning die a new dimension of ceramic fibers isavailable. A wide variety of fiber combinations are available since thesols are fed as independent streams to the feed cavity and the orifice.The first stream can comprise a single metal oxide sol, a blend of twoor more major component sols, or any combination of minor additives toprovide, for example, color. In the same manner, the second stream cancomprise any of the same combinations. A wide range of combinations areavailable to impart a wide range of properties to meet specialty needsof ceramic fibers. For example, a blend of high modulus with lowermodulus base sol provides a bicomponent fiber with reduced friability;that is, the fiber can withstand the flex and bends of textile weavingyet provide a higher strength fabric. Bicolor fibers can be provided forapplications requiring color coding.

Ceramic fibers which are magnetic or attractive to a magnet are highlydesirable, for example, as magnetic filter media to remove ironparticulate contaminants in high purity ceramic slurries. High surfacearea, low pressure-drop fiber catalyst systems are highly desirable, forexample, as catalytic distillation sections in distillation columns.When sols of these catalytic and magnetic type materials are spun intofibers, they are often too weak and friable to be converted into usefulforms. The bicomponent fiber provides an unusual opportunity to developthese fiber product systems. Strong fibers, e.g., alumina-boria-silica3-1-2 or alumina-silica (mullite) fibers provide a good carrier for thelow strength magnetic fibers (e.g., barium titanate), cyatalytic fibers(e.g., platinum cermet), non-magnetic fibers (e.g., tungsten cermet), ormagnetic attractive fibers (e.g., nickel).

Objects and advantages of this invention are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this invention.

EXAMPLES 1-7

The general method for obtaining bicomponent fibers of variouscomponents is illustrated as follows:

A sol consisting of 200 g basic aluminum acetate in 400 mL water and 86g aqueous colloidal silica dispersion resulting in an oxide equivalentof 3Al₂ O₃.1B₂ O₃.2SiO₂ was cospun with a like sol containing 1 weightpercent Fe₂ O₃ to provide a color source as an aid to identify the dualcomposition fiber. The two sols were placed in a spinnerette cup having127 micrometer (5 mil) orifices arranged in a straight line. Eachspinning sol was placed on opposite sides of the orifice line in thespinnerette cup. The spinnerette unit was assembled and nitrogen gaspressure at 14.7 kPa (100 psi) was applied. The formed fibers werecollected on a wood stick at the bottom of the spinning tower which wasequipped with heat lamps to dry the falling fibers.

The collected fibers were fired from room temperature (RT) to 800° C. inabout two hours. The fibers were then observed under a microscopeshowing a single fiber, part white and part black as shown in the blackand white photo, FIG. 5. Because the spinning sols were essentially ofthe same composition, there was virtually no crimping.

Examples of bicomponent fibers that were made using the method of thisinvention are shown in TABLE I below:

                                      TABLE I                                     __________________________________________________________________________    Components*            Orifice size-                                                                        Fiber                                           Ex.                                                                              A         B         micrometer                                                                           comments**                                      __________________________________________________________________________    1  A-B-S 3-1-2                                                                             A-B-S 3-1-2 +                                                                           127    rust brown                                                   1% Fe.sub.2 O.sub.3                                                                            no crimp                                        2  A-B-S 3-1-2                                                                             A-S + 2% B.sub.2 O.sub.3 +                                                              152    light green                                                  1Cr.sub.2 O.sub.3                                                                              high crimp                                      3  A-S + 2% B.sub.2 O.sub.3                                                                A-S + 2% B.sub.2 O.sub.3 +                                                              152    light green                                        1% Cr.sub.2 O.sub.3                                                                     1% CuO           light crimp                                     4  Zr-S      A-B-S 3-1-2                                                                              76    white                                                                         light crimp                                     5  A-S + 2% B.sub.2 O.sub.3 +                                                              A-S +     152    rust brown                                         20% Fe.sub.2 O.sub.3                                                                    A-B-S 3-1-12     very tight                                                                    crimp                                           6  A-B-S 3-1-2                                                                             A-S + Cr.sub.2 O.sub.3                                                                   76    light green                                                  A-B-S 3-1-12     high crimp                                      7  A-S + 2% B.sub.2 O.sub.3 +                                                              A-S + 2% B.sub.2 O.sub.3                                                                152    light blue                                         1% CoO                                                                     __________________________________________________________________________     *A-B-S 31-2 aluminaboria-silica 3:1:2 (molar ratio)                           A-B-S 31-12 aluminaboria-silica 3:1:12 (molar ratio)                          A-S aluminasilica (mullite) 3:2 (molar ratio)                                 Zr-S zirconiasilica 1:1 (molar ratio)                                         **Crimping is caused by differences in shrinkage between the two              components during drying and/or firing processes.                        

EXAMPLE 8

An Al₂ O₃ sol was co-spun with a 50:50 Al₂ O₃ -ZrO₂ sol. The alumina solwas prepared by adding 68 g lactic acid to 1100 g aluminum formoacetateand concentrating the sol to the spinning sol viscosity. The 1:1 Al₂ O₃:ZrO₂ sol was prepared by mixing 492 g zirconium acetate into 1122 galuminum formoacetate with a further addition of 70 g lactic acid. Thesol precursor was concentrated to the spinning viscosity. The fiberswere fired to 700° C. and held for 0.5 hour. As observed in an opticalmicroscope, bicomponent fibers were formed; the shrinkage was minimaland low crimping was seen.

EXAMPLE 9 Basic Sol with Additives as One Component Fiber

A sol consisting of 70% by weight A-B-S 3-1-2 and 30% by weight Ni wasprepared as follows (about 8.5 weight percent oxides)

100 g BAA (basic aluminum acetate, 7 weight percent 3Al₂ O₃ :1B₂ O₃oxide equivalent based on drying and calcination to the oxide state)

3.0 g lactic acid (85% by weight)

6.8 g silica sol, (Nalco 1034-A™, Nalco Chemical Co., Chicago, Ill.)

3.0 g dimethyl formamide

were mixed together to form the basic A-B-S 312 sol. To this mixture wasadded 17.34 grams of nickel acetate.4 H₂ O which was dissolved in 50 mlwater.

The total mixture was filtered through a 0.3 micrometer Balston™ filtertube (Balston, Inc., Lexington, Mass.) cartridge filter, thenconcentrated in a Rotovapor™-R rotating evacuated flask (Buchi,Switzerland) to a viscous sol of about 50,000 centipoise. This sol wasreadily spun using 29.4 kPa (200 psi) N₂ pressure and a 30 hole, 76micrometer (3 mil) spinnerette. The fiber was light green in color. Thissol was co-spun with A-B-S 3-1-2 which when heated in a hydrogenatmosphere resulted in a fiber which was attractive to a magnet.

EXAMPLE 10 Bicomponent Fibers of A-B-S 312/A-B-S 312+30% Fe₂ O₃

1. 541.2 g of a 7 percent aqueous solution of basic aluminum acetate(BAA), where 7% means oxide equivalent based on calcining of driedmaterial which will have a ratio or 3 mole alumina to 1 mole boria

2. 15.0 g of 85 weight percent lactic acid

3. 35.7 g silica sol (Nalco™ 1034A, Nalco Chemical Co., Oak Brook, Ill.)

4. 14.5 g dimethylformamide (DMF)

The mixture was prepared by mixing the above materials in the order(1+2+3+4). The resulting mixture was filtered through a 0.3 Balston™cartridge (Balston, Inc., Lexington, Mass.) and 1 Millipore™ filtersinto a round bottomed flask to be concentrated in a Rotavapor-R (waterbath was 32°-35° C.) to a viscous sol of about 86,000 centipoiseviscosity.

The other half of the mixture (equivalent to 25 g oxide content) wasstirred into 500 g of iron acetate sol (1.5% Fe₂ O₃); the resultingmixture was then filtered through 0.3 and 1 micrometer filters toanother round flask to be concentrated in a Rotovapor-R to a viscous solof about 97,000 cps viscosity. Both viscous sols were then filled into a101 micrometer (4 mil)×20 hole spinnerette. The holes were in a row atthe center of the spinnerette. A thin glass baffle (with a small spaceabove the holes) was used as the boundary of both sols. Both sols werespun into fibers 29.4 kPa (200 psi lbs. pressure) which were fired:

In air, from room temperature to 850° C. in 3 hrs. and soaked at 850° C.for 1/2 hr. The fiber showed no magnetic properties.

In 100% H₂ atmosphere: half of the air fired fibers above was fired fromroom temperature to 900° C. in 45 minutes, soaked at 900° C. for 1/2 hr.then cooled to room temperature. The fiber had magnetic properties, thatis, was attracted to a permanent magnet.

FIGS. 3, 4, and 5 show an air dried and hydrogen fired bicomponent fiber30, with component fibers 32 and 34 being in laminated longitudinallyextended configuration.

EXAMPLES 11 AND 12

A magnetic bicomponent fiber was prepared by co-spinning an A-B-S 3-1-2sol with a barium titanate precursor sol. The barium titanate precursorsol was prepared from a water solution of barium acetate plus anhydrousferrous acetate in a mole ratio of 1Ba:12Fe.

A second bicomponent fiber was prepared as described immediately aboveexcept that instead of ferrous acetate, FeCl₃.6H₂ O was precipitatedwith NH₄ OH in an aqueous solution.

To aid in fiberization, a small amount of polyvinylpyrrolidone(PYP-K30™, GAF), dissolved in water, was stirred into each of the twosols described above. Upon concentration, the sols became viscous andtacky. Continuous bicomponent fibers were spun from these concentratesand A-B-S 3-1-2. These bicomponent fibers were magentic.

High loft webs of these fibers can be used to remove iron contaminantsfrom a ceramic slurry.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention, and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

I claim:
 1. A continuous, bicomponent, non-vitreous ceramic fiber whichis circular, oval, or rod-like in cross-section comprising two ceramiccomponents in a longitudinally-extending side-by-side relationshipwherein each of the components is derived from a different compatiblefiber-forming aqueous-based ceramic precursor liquid, and wherein saidcomponents are independently selected from the group consisting oftitania, alumina-chromia metal (IV) oxide, alumina-silica, thoria-silicametal (III) oxide, alumina-boria, alumina-boria-silica, zirconia-silica,and combinations thereof.
 2. The fiber according to claim 1 wherein eachof the components provide different ceramic compositions.
 3. The fiberaccording to claim 1 which is unfired.
 4. The fiber according to claim 1which is fired.
 5. A continuous, bicomponent, non-vitreous ceramic fiberwhich is circular, oval, or rod-like in cross-section comprising twoceramic components in a longitudinlly-extending side-by-siderelationship wherein each of the components is derived from a differentcompatible fiber-forming aqueous-based ceramic precursor liquid, andwherein said components are independently selected from the groupconsisting of titania, alumina-chromia metal (IV) oxide, alumina-silica,thoria-silica metal (III) oxide, alumina-boria, alumina-boria-silica,zirconia-silica, and combinations thereof, and at least one of saidceramic components also contains 0.05 to 25 weight percent based on thetotal weight of the component in the fiber of at least one otherwater-soluble metal compound calcinable to metal oxide.
 6. A continuous,bicomponent, non-vitreous ceramic fiber which is circular, oval, orrod-like in cross-section comprising two ceramic components in alongitudinally-extending side-by-side relationship wherein each of thecomponents is derived from a different compatible fiber-formingaqueous-based ceramic precursor liquid, and wherein said components areindependently selected from the group consisting of titania,alumina-chromia metal (IV) oxide, alumina-silica, thoria-silica metal(III) oxide, alumina-boria, alumina-boria-silica, zirconia-silica, andcombinations thereof, and each of said ceramic components also contains0.05 to 25 weight percent based on the total weight of the component inthe fiber of at least one additional water-soluble metal compoundcalcinable to metal oxide.
 7. The fiber according to claim 1 whereinsaid components are present in a volume ratio range of 50:50 to 10:90.8. The fiber according to claim 1 wherein said components are present ina volume range ratio of 40:60 to 60:40.
 9. The fiber according to claim1 which is crimped.
 10. The fiber according to claim 5 which is a mono-or bicolor filament.
 11. The fiber according to claim 6 which is abicolor filament.
 12. The fiber according to claim 1 which is magnetic.13. The fiber according to claim 1 which is attractive to a magnet. 14.A continuous, bicomponent, non-vitreous ceramic fiber which is circular,oval, or rod-like in cross-section comprising two ceramic components ina longitudinally-extending side-by-side relationship wherein each of thecomponents is derived from a different compatible fiber-formingaqueous-based ceramic precursor liquid, and wherein said components areindependently selected from the group consisting of titania,alumina-chromia metal (IV) oxide, alumina-silica, thoria-silica metal(III) oxide, alumina-boria, alumina-boria-silica, zirconia-silica, andcombinations thereof, and both of said ceramic components also containreducible metal oxides, which fiber is convertible to a cermet/cermetfiber when fired in a reducing atmosphere.
 15. A yarn comprising thefiber according to claim
 1. 16. A woven fabric comprising the fiberaccording to claim
 1. 17. A nonwoven web comprising the fiber accordingto claim
 1. 18. The nonwoven web according to claim 17 useful incatalytic distillation processes.
 19. A mat comprising the crimped fiberaccording to claim 9.